Polarizing plate and liquid crystal display apparatus

ABSTRACT

An object of the present invention is to provide a thin polarizing plate exhibiting a high contrast ratio. A polarizing plate of the present invention includes: a polarizer; a first protective layer provided on one side of the polarizer; and a second protective layer provided on the other side of the polarizer, wherein the first protective layer has a function of separating incident light into two polarized light components perpendicular to each other, transmitting one polarized light component, and reflecting the other polarized light component. Such a polarizing plate can exhibit a high contrast ratio, for example, in the case of being used in a liquid crystal display apparatus.

TECHNICAL FIELD

The present invention relates to a polarizing plate including aprotective layer that has a function of separating incident light intotwo polarized light components perpendicular to each other, andtransmitting one polarized light component and reflecting the otherpolarized light component.

BACKGROUND ART

A liquid crystal display apparatus (hereinafter, referred to as “LCD”)is a device for displaying characters and images, using electroopticalproperties of liquid crystal molecules, and has been widely used forcell phones, notebook computers, liquid crystal televisions, and thelike. The LCD generally uses a liquid crystal panel, in which polarizingplates are arranged on both sides of a liquid crystal cell, and candisplay a black image under no voltage application, for example, in anormally black mode (e.g., see Patent Document 1). Recently, as thedefinition of the LCD has been increased, and the range of applicationsthereof has been enlarged, there is a demand for a thin polarizing plateexhibiting a high contrast ratio, capable of drawing characters andimages more clearly.

Patent Document 1: JP 9-269504 A

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

An object of the present invention is to provide a thin polarizing plateexhibiting a high contrast ratio.

Means for Solving the Problems

As a result of an intensive study, the inventors have found that theabove-described object can be achieved according to the polarizing platedescribed below so as to complete the present invention.

That is, a polarizing plate of the present invention includes: apolarizer; a first protective layer provided on one side of thepolarizer; and a second protective layer provided on the other side ofthe polarizer, wherein the first protective layer has a function ofseparating incident light into two polarized light componentsperpendicular to each other, transmitting one polarized light component,and reflecting the other polarized light component.

In a preferred embodiment, a transmission axis direction of thepolarizer and a transmission axis direction of the first protectivelayer are substantially parallel to each other

In a preferred embodiment, the polarizer includes a stretched film of apolyvinyl alcohol-based resin containing a dichroic pigment.

In a preferred embodiment, the polarizer includes a solidified layer ora cured layer of an aligned lyotropic liquid crystal.

In a preferred embodiment, the lyotropic liquid crystal contains anazo-based pigment, an anthraquinone-based pigment, a perylene-basedpigment, an indanthrone-based pigment, an imidazole-based pigment, or amixture thereof.

In a preferred embodiment, the first protective layer includes alaminate containing a thermoplastic resin layer (A) and a thermoplasticresin layer (B).

In a preferred embodiment, the first protective layer includes thethermoplastic resin layer (A) and the thermoplastic resin layer (B)provided alternately.

In a preferred embodiment, the thermoplastic resin layer (A)substantially exhibits anisotropy, and the thermoplastic resin layer (B)substantially exhibits isotropy.

In a preferred embodiment, an in-plane birefringent index (Δn_(xy)[590])at a wavelength of 590 nm of the thermoplastic resin layer (A) is 0.05or more.

In a preferred embodiment, a refractive index (ny_(A)) in a fast axisdirection of the thermoplastic resin layer (A) and a refractive index(ny_(B)) in a fast axis direction of the thermoplastic resin layer (B)are substantially the same.

In a preferred embodiment, the thermoplastic resin layer (A) contains apolyethylene terephthalate-based resin, a polytrimethyleneterephthalate-based resin, a polybutylene terephthalate-based resin, apolyethylene-naphthalate-based resin, a polybutylene naphthalate-basedresin, or a mixture thereof.

In a preferred embodiment, the thermoplastic resin layer (B) contains apolystyrene-based resin, a polymethyl methacrylate-based resin, apolystyrene glycidyl methacrylate-based resin, or a mixture thereof.

In a preferred embodiment, an in-plane retardation value (Re[590]) at awavelength of 590 nm of the second protective layer is 10 nm to 400 nm.In a preferred embodiment, a thickness direction retardation value(Rth[590]) at a wavelength of 590 nm of the second protective layer is10 nm to 800 nm.

In a preferred embodiment, the second protective layer contains at leastone kind of resin selected from the group consisting of acellulose-based resin, a norbornene-based resin, a polyimide-basedresin, a polyester-based resin, and an acrylic resin.

In a preferred embodiment, the polarizing plate further includes anadhesion layer on a side of the second protective layer opposite to aside having the polarizer. In a preferred embodiment, the polarizingplate further includes a retardation layer on a side of the secondprotective layer opposite to a side having the polarizer. In a preferredembodiment, the retardation layer has a function of compensating aliquid crystal cell optically.

According to another aspect of the present invention, a liquid crystaldisplay apparatus is provided. The liquid crystal display apparatusincludes the polarizing plate according to any one of claims 1 to 18.

EFFECTS OF THE INVENTION

In the polarizing plate of the present invention, the second protectivelayer has a function of separating incident light into two polarizedlight components perpendicular to each other, and transmitting onepolarized light component and reflecting the other polarized lightcomponent. Therefore, the polarizing plate of the present invention isthinner than a conventional polarizing plate, and when the polarizingplate of the present invention is used in a liquid crystal displayapparatus, much higher contrast ratio may be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A schematic cross-sectional view of a polarizing plate accordingto a preferred embodiment of the present invention.

FIG. 2 A schematic perspective view of a polarizing plate according to apreferred embodiment of the present invention.

DESCRIPTION OF SYMBOLS

-   -   1 absorption axis direction    -   2 transmission axis direction    -   3 reflection axis direction    -   4 transmission axis direction    -   5 slow axis direction    -   10 polarizer    -   21 first protective layer    -   22 second protective layer    -   50 polarizing plate

BEST MODE FOR CARRYING OUT THE INVENTION Definitions of Terms andSymbols

Definitions of terms and symbols in the specification of the presentinvention are described below.

(1) Transmittance of a Polarizing Plate

Transmittance (T) refers to a Y value of tristimulus value based on thetwo degree field of JIS Z 8701-1995.

(2) Refractive Index (nx, ny, nz):

Symbol “nx” refers to a refractive index in a direction providing amaximum in-plane refractive index (that is, a slow axis direction),symbol “ny” refers to a refractive index in a direction perpendicular tothe slow axis in the plane (that is, a fast axis direction), and symbol“nz” refers to a refractive index in a thickness direction.

(3) In-Plane Retardation Value:

The term “in-plane retardation value (Re[λ])” refers to an in-planeretardation value measured at 23° C. by using light of a wavelength of λnm. Re[λ] is obtained from Re[λ]=(nx−ny)×d, where d (nm) represents athickness of a sample.

(4) Thickness Direction Retardation Value:

The term “thickness direction retardation value (Rth[λ])” refers to athickness direction retardation value measured at 23° C. by using lightof a wavelength of λnm. Rth[λ] is obtained from Rth[λ]=(nx−nz)×d, whered (nm) represents a thickness of a sample.

(5) Thickness Direction Birefringent Index:

The term “thickness direction birefringent index (Δn_(xz)[λ])” refers toa value calculated from an expression; Rth[λ]/d. In the expression,Rth[λ] represents a thickness direction retardation value measured at23° C. by using light of a wavelength of λ nm and d (nm) represents athickness of a film.

(6) Nz Coefficient:

The term “Nz coefficient” refers to a value calculated from anexpression; Rth[590]/Re[590].

(7) In the present specification, the phrase “nx=ny” or “ny=nz” includesnot only a case where these are exactly equal but also a case wherethese are substantially equal. Thus, for example, the description“nx=ny” includes a case where Re[590] is less than 10 nm.

(8) In the present specification, the phrase “substantiallyperpendicular” includes a case where an angle between two optical axesis 90°±2°, preferably 90°±1°. The phrase “substantially parallel”includes a case where an angle between two optical axes is 0°±2°,preferably 0°±1°.

(9) In the present specification, for example, the subscript “1”represents a first protective layer and the subscript “2” represents asecond protective layer.

<A. Outline of Polarizing Plate>

FIG. 1 is a schematic cross-sectional view of a polarizing plate of thepresent invention. A polarizing plate 50 includes a polarizer 10, afirst protective layer 21 provided on one side of the polarizer 10, anda second protective layer 22 provided on the other side of the polarizer10. The first protective layer 21 has a function of separating incidentlight into two polarized light components perpendicular to each other,and transmitting one polarized light component and reflecting the otherpolarized light component.

The thickness of the polarizing plate is preferably 10 μm to 500 μm, andmore preferably 30 μm to 300 μm.

For a practical use, any adhesion layer may be provided in between theconstituent members of the polarizing plate 50. The term “adhesionlayer” refers to a layer that connects surfaces of adjacent members andintegrate them with a practically sufficient adhesion strength and foran adhesion time. Examples of a material for forming the adhesion layerinclude an adhesive, a pressure-sensitive adhesive, and an anchor coatagent. The adhesion layer may have a multi-layered structure in which ananchor coat agent is formed on the surface of an adherend and anadhesive layer or a pressure-sensitive adhesive layer is formed thereon.Further, the adhesion layer may be a thin layer (which may be referredto as hair line) which cannot be recognized with naked eyes.

FIG. 2 is a schematic perspective view of the polarizing plate of thepresent invention in a preferred embodiment. The first protective layer21 has an axis direction (transmission axis direction 4) in which one ofthe two separated polarized light components perpendicular to each otheris transmitted and an axis direction (reflection axis direction 3) inwhich the other polarized light component is reflected. The polarizer 10has an absorption axis direction 1 and a transmission axis direction 2.The second protective layer has a slow axis direction 22. Thetransmission axis direction 2 of the polarizer 10 and the transmissionaxis direction 4 of the first protective layer 21 are substantiallyparallel to each other. In the illustrated example, the absorption axisdirection 1 of the polarizer 10 and the slow axis direction 5 of thesecond protective layer 22 are substantially perpendicular to eachother. However, they may be substantially parallel to each other or mayhave a relationship in which they are neither perpendicular norparallel. Hereinafter, constituent members of the present invention willbe described in detail; however, the present invention is not limited toonly the following particular embodiments.

<B. Polarizer>

As the polarizer used in the present invention, any suitable one may beselected. Preferably, the polarizer has a function of separatingincident light into two polarized light components perpendicular to eachother, and transmitting one polarized light component and absorbing theother polarized light component.

The light transmittance (T) of the polarizer is preferably 40% to 45%,more preferably 41% to 45%. The polarization degree (P) of the polarizeris preferably 99% or more, more preferably 99.5% or more. By setting Tand P in the above range, a liquid crystal display apparatus with a muchhigher contrast ratio may be obtained.

The light transmittance (T) and the polarization degree (P) may bedetermined by using a spectrophotometer “DOT-3” (product name,manufactured by Murakami Color Research Laboratory). The polarizationdegree may be determined by: measuring a parallel transmittance (H₀) anda perpendicular transmittance (H₉₀) of the polarizing plate; and usingthe following equation. Polarization degree(%)={(H₀−H₉₀)/(H₀+H₉₀)}^(1/2)×100. The parallel transmittance (H₀)refers to a transmittance of a parallel laminate polarizing plateproduced by laminating two identical polarizing plates such thatrespective absorption axes are parallel to each other. The perpendiculartransmittance (H₉₀) refers to a transmittance of a perpendicularlaminate polarizing plate produced by laminating two identicalpolarizing plates such that respective absorption axes are perpendicularto each other. The transmittance refers to a Y value of a tristimulusvalue based on a two-degree field of JIS Z 8701-1995.

In one embodiment, the polarizer is a stretched film of a polyvinylalcohol-based resin containing a dichroic pigment. In the presentspecification, the “dichroic pigment” refers to a pigment whosetransition moment in a major axis direction of the pigment molecule islarger than that in a minor axis direction, or whose transmission momentin a minor axis direction is larger than that in a major axis direction.

For example, as described in JP 2004-341515 A, the stretched film may beobtained by allowing a polymer film containing a polyvinyl alcohol-basedresin as a main component to swell, and thereafter, stretching thepolymer film 4 to 6 times the original length while dyeing the polymerfilm with a dichroic pigment.

The thickness of the stretched film of a polyvinyl alcohol-based resincontaining a dichroic pigment is preferably 10 μm to 100 μm, and morepreferably 10 μm to 50 μm.

Iodine, a dichroic dye, and the like may be exemplified as the dichroicpigment. Examples of the dichroic dye include Red BR, Red LR, Red R,Pink LB, Rubin BL, Bordeaux GS, Sky Blue LG, Lemon Yellow, Blue BR, Blue2R, Navy RY, Green LG, Violet LB, Violet B, Black H, Black B, Black GSP,Yellow 3G, Yellow R, Orange LR, Orange 3R, Scarlet GL, Scarlet KGL,Congo Red, Brilliant Violet BK, Supra Blue G, Supra Blue GL, SupraOrange GL, Direct Sky Blue, Direct Fast Orange S, and Fast Black.

The content of the dichroic dye is preferably 1% by weight to 10% byweight. In the case where the dichroic dye is iodine, the content ofiodine is preferably 1.5% by weight to 5.0% by weight. By setting thecontent of iodine in the polarizer in the above range, a transmittanceand a polarization degree in preferred ranges may be obtained.

Preferably, the stretched film of a polyvinyl alcohol-based resincontaining a dichroic dye further contains potassium and/or boron. Thecontent of potassium is preferably 0.2% by weight to 1.0% by weight. Thecontent of boron is preferably 0.5% by weight to 3.0% by weight. Bysetting the content of potassium and the content of boron in the aboveranges, a polarizing plate with a transmittance in a preferred range anda high polarization degree may be obtained.

The polyvinyl alcohol-based resin may be obtained by saponifying a vinylester-based polymer obtained by polymerizing a vinyl ester-basedmonomer. The saponification degree of the polyvinyl alcohol-based resinis preferably 95.0% by mol to 99.9% by mol. The saponification degreemay be obtained in accordance with JIS K 6726-1994. By using a polyvinylalcohol-based resin having a saponification degree in the above range, apolarizer excellent in durability may be obtained.

In another embodiment, the polarizer is a solidified layer or curedlayer of an aligned lyotropic liquid crystal. In the specification ofthe present invention, the term “lyotropic liquid crystal” refers to aliquid crystal that causes phase transition of isotropic phase/liquidcrystal phase by concentration change of solute (liquid crystalcompound) as a major factor. The term “solidified layer” refers to alayer obtained by cooling and solidifying a softened, molten, orsolution-state liquid crystalline composition. The term “cured layer”refers to a layer obtained by cross-linking part or all of the liquidcrystalline composition through heat, a catalyst, light, and/orradiation into an insoluble and infusible, or hardly soluble and hardlyfusible state.

The solidified layer or cured layer of the aligned lyotropic liquidcrystal is excellent in absorption dichroism, so that the solidifiedlayer or cured layer may be formed thin. The thickness of the solidifiedlayer or cured layer is preferably 0.1 μm to 10 μm, and more preferably0.1 μm to 5 μm.

The solidified layer or cured layer of the aligned lyotropic liquidcrystal may be obtained, for example, by mixing a lyotropic liquidcrystal with a solvent (for example, water), preparing a solutionexhibiting a nematic liquid crystal phase, and casting the solution ontothe surface of a base, followed by drying.

The concentration of the solution may be appropriately adjusted in arange in which the solution exhibits a liquid crystal phase, dependingupon the kind of a lyotropic liquid crystal to be used. Theconcentration of the lyotropic liquid crystal of the solution ispreferably 5% by weight to 40% by weight.

Examples of a liquid crystal phase exhibited by the lyotropic liquidcrystal in a solution state include a nematic liquid crystal phase, asmectic liquid crystal phase, and a cholesteric liquid crystal phase.The nematic liquid crystal phase is preferred. These liquid crystalphases may be checked and identified based on an optical pattern of aliquid crystal phase observed with a polarization microscope.

The lyotropic liquid crystal preferably absorbs light with anywavelength in the range of 400 nm to 780 nm. The lyotropic liquidcrystal, for example, contains a dichroic pigment according to a desiredabsorption wavelength. Examples of the dichroic pigment include,according to classification by chemical structures, azo-based pigments,anthraquinone-based pigments, perylene-based pigments, indanthrone-basedpigments, imidazole-based pigments, indigoid-based pigments,oxazine-based pigments, phthalocyanine-based pigments,triphenylmethane-based pigments, pyrazolone-based pigments,stilbene-based pigments, diphenylmethane-based pigments,naphthoquinone-based pigments, methocyanine-based pigments,quinophthalone-based pigments, xanthene-based pigments, alizarin-basedpigments, acridine-based pigments, quinoneimine-based pigments,thiazole-based pigments, methine-based pigments, nitro-based pigments,and nitroso-based pigments. In the present invention, in order to obtainblack polarizer, plural dichroic pigments having different adsorptionspectra from one another are preferably mixed for use.

Preferably, the lyotropic liquid crystal contains azo-based pigments,anthraquinone-based pigments, perylene-based pigments, indanthrone-basedpigments, imidazole-based pigments, or a mixture thereof. Such compoundshave rigidity and anisotropy required for expressing a liquid crystalphase, exhibits a stable liquid crystal phase in a solution, and absorbslight in a wavelength region in a wide range of 400 nm to 780 nm.

Preferably, the lyotropic liquid crystal is a polycyclic compoundcontaining a —SO₃M group and/or a —COOM group (herein, M represents acounter ion). Particularly preferably, the lyotropic liquid crystal is apolycylic compound containing a —SO₃M group in order to enhance thesolubility in water. The polycyclic compound becomes likely to form anassociation with a high order in a solution by containing a —SO₃M groupand/or a —COOM group. Therefore, a film formed of such a solution alsoexhibits a high alignment property, and as a result, a polarizerexcellent in optical properties may be obtained.

The above-mentioned M is a counter ion, and is preferably a hydrogenatom, an alkali metal atom, an alkaline-earth metal atom, a metal ion,or a substituted or unsubstituted ammonium ion. Examples of the metalions include Ni²⁺, Fe³⁺, Cu²⁺, Ag⁺, Zn²⁺, Al³⁺, Pd²⁺, Cd²⁺, Sn²⁺, Co²⁺,Mn²⁺, and Ce³⁺. For example, in the case where a polarizer is formed ofan aqueous solution, a group that enhances solubility in water isselected first as the M, and after the formation of a film, awater-insoluble or slightly-soluble group may substitute for theselected group so as to enhance the water resistance of the polarizer.

If the polycyclic compound is used, a polarizer having a transmissionaxis substantially parallel to an application direction may be produced.Such a polarizer may be attached to the first protective layer having atransmission axis direction substantially parallel to a stretchingdirection by roll-to-roll, so that the productivity of a polarizingplate may be enhanced greatly.

As a method of introducing a sulfone group into the polycyclic compound(sulfonation), for example, there is a method of allowing sulfuric acid,chlorosulfonic acid, or fuming sulfuric acid to act on an organiccompound, and substituting a sulfonic group for hydrogen at a core. Asalt of the organic compound is obtained by substituting monovalentcation such as a lithium ion, a sodium ion, a potassium ion, a cesiumion, or an ammonium ion for a dissociable hydrogen atom in the acidgroup.

As the lyotropic liquid crystal, in addition to the above-mentionedcompounds, compounds described in, for example, JP2006-047966A,JP2005-255846A, JP2005-154746A, JP2002-090526 A, JP08-511109A,JP2004-528603A, JP2004-528603A, JP2004-528603 A, and the like may beused.

<C. First Protective Layer>

The first protective layer used in the present invention has a functionof separating incident light into two polarized light componentsperpendicular to each other, and transmitting one polarized lightcomponent and reflecting the other polarized light component. Further,the first protective layer can prevent a polarizer from shrinking orexpanding. The first protective layer has an axis direction(transmission axis direction) in which one polarized light component ofthe two separated polarized light components perpendicular to each otheris transmitted and an axis direction (reflection axis direction) inwhich the other polarized light component is reflected.

The first protective layer is used for enhancing a brightness (whitebrightness) in the case where a white image is displayed on a liquidcrystal display apparatus. A conventional polarizing plate can increasea white brightness by using a brightness enhancing film; however, at thesame time, a brightness in the case where a black image is displayed(black brightness) is increased, and consequently, a high contrast ratioin a front direction cannot be obtained. The polarizing plate with aconfiguration of the present invention can minimize the increase in ablack brightness while increasing a white brightness, so that a highcontrast ratio in a front direction may be obtained.

A reflectivity (R_(1x)[590]) of the first protective layer in thereflection axis direction at a wavelength of 590 nm is preferably 60% ormore, and more preferably 70% or more. A reflectivity (R_(1y)[590]) ofthe first protective layer in the transmission axis direction ispreferably less than 50%, and more preferably 30% or less. Thereflectivity in the reflection axis direction and the transmission axisdirection is measured with a spectrophotometer having an integratingsphere accessory and a polarizer under the condition that the reflectionaxis direction and the transmission axis direction of the firstprotective layer are respectively provided so as to be parallel to apolarization electric field vector of incident light.

A transmittance (T_(1y)[590]) of the first protective layer in thetransmission axis direction is preferably 60% or more, and morepreferably 70% or more. The transmittance (T_(1x)[590]) of the firstprotective layer in the reflection axis direction is preferably lessthan 50%, and more preferably 30% or less. The transmittance in thetransmission axis direction and the reflection axis direction ismeasured with a spectrophotometer having an integrating sphere accessoryand a polarizer under the condition that the transmission axis directionand the reflection axis direction of the first protective layer arerespectively provided so as to be parallel to a polarization electricfield vector of incident light.

The first protective layer preferably is allowed to adhere to thepolarizer via an adhesion layer. Preferably, the transmission axisdirection of the polarizer and the transmission axis direction of thefirst protective layer are substantially parallel to each other. Thatis, the absorption axis direction of the polarizer and the transmissionaxis direction of the first protective layer are substantiallyperpendicular to each other.

Preferably, the first protective layer is a laminate including athermoplastic resin layer (A) and a thermoplastic rein layer (B).Typically, the first protective layer is a layer in which thethermoplastic resin layers (A) and the thermoplastic resin layers (B)are placed alternately (ABABAB . . . ). The number of the layersconstituting the first protective layer is preferably 10 to 900, andmore preferably 50 to 700. The total thickness of the first protectivelayer is preferably 20 μm to 800 μm.

Preferably, the thermoplastic resin layer (A) exhibits anisotropyoptically. A birefringent index (Δn_(A)) in a plane of the thermoplasticresin (A) is preferably 0.05 or more, more preferably 0.1 or more, andparticularly preferably 0.15 or more. From the viewpoint of opticaluniformity, the upper limit value of the Δn_(A) is preferably 0.2.Herein, the Δn_(A) represents a difference (nx_(A)−ny_(A)) between arefractive index (nx_(A)) in a slow axis direction and a refractiveindex (ny_(A)) in a fast axis direction.

The thermoplastic resin layer (B) preferably exhibits isotropyoptically. A birefringent index (Δn_(B)) in a plane of the thermoplasticresin (B) is preferably 5×10⁻⁴ or less, 1×10⁻⁴ or less, and particularlypreferably 0.5×10⁻⁴ or less. The lower limit value of the Δn_(B) ispreferably 0.01×10⁻⁴. Herein the Δn_(B) represents a difference(nx_(B)−ny_(B)) between nx_(B) (refractive index in a slow axisdirection) and ny_(B) (refractive index in a fast axis direction).

ny_(A) of the thermoplastic resin layer (A) and ny_(B) of thethermoplastic resin layer (B) preferably are substantially the same. Theabsolute value of a difference between ny_(A) and ny_(B) is preferably5×10⁻⁴ or less, 1×10⁻⁴ or less, and particularly preferably 0.5×10⁻⁴ orless. The first protective layer having such optical properties isexcellent in a function of reflecting a polarized light component. Inthe case where the thermoplastic resin layer (B) is completelyisotropic, the ny_(B) and ny_(A) represent refractive indices in thesame direction.

The first protective layer is produced by co-extruding two kinds ofresins and stretching the extruded film, for example, as described in JP2000-506989 A. According to such a method, the thermoplastic resin layer(A) exhibits anisotropy optically, and a slow axis substantiallyparallel to the stretching direction is expressed. Then, by settingstretching conditions appropriately, the ny_(A) of the thermoplasticresin layer (A) and the ny_(B) of the thermoplastic resin layer (B) maybe set to be substantially the same. Consequently, a layer having areflection axis parallel to the stretching direction may be produced.

As a resin forming the thermoplastic resin layer (A), any suitable onemay be selected. The thermoplastic resin layer (A) preferably contains apolyethylene terephthalate-based resin, a polytrimethyleneterephthalate-based resin, a polybutylene terephthalate-based resin, apolyethylene naphthalate-based resin, a polybutylene naphthalate-basedresin, or a mixture thereof. These resins are excellent in an expressionproperty of a birefringence by stretching and stability of birefringenceafter stretching.

As the thermoplastic resin layer (B), any suitable one may be selected.The thermoplastic resin layer (B) preferably contains apolystyrene-based resin, a polymethylmethacrylate-based resin, apolystyrene glycidyl methacrylate-based resin, or a mixture thereof. Inorder to enhance a refractive index, a halogen group such as chlorine,bromine, and iodine may be introduced into the resin. Alternatively, theresin may contain any additive so as to adjust a refractive index.

As the first protective layer, a commercially available brightnessenhancing film may be used as it is. An example of the commerciallyavailable brightness enhancing film includes Vikuiti DBEF seriesmanufactured by Sumitomo 3M Ltd., and the like.

<D. Second Protective Layer>

The second protective layer used in the present invention is provided ona side of the polarizer opposite to the side where the first protectivelayer is provided. The second protective layer can prevent the polarizerfrom shrinking and expanding. The second protective layer is preferablyadhered to the polarizer via an adhesion layer.

The second protective layer may be a single layer or a laminate formedof a plurality of layers. The thickness of the second protective layeris preferably 10 μm to 200 μm. The transmittance (T₂[590]) at awavelength of 590 nm of the second protective layer is preferably 90% ormore.

In-plane and thickness direction retardation values of the secondprotective layer may be set appropriately depending upon the purpose.The in-plane retardation value (Re₂[590]) at a wavelength of 590 nm ofthe second protective layer is preferably 10 nm to 400 nm, and morepreferably 40 nm to 400 nm. The thickness direction retardation value(Rth₂[590]) at a wavelength of 590 nm of the second protective layer ispreferably 10 nm to 800 nm, and more preferably 40 nm to 800 nm. Asdescribed above, by imparting a retardation to the second protectivelayer, a high contrast ratio may be obtained not only in a frontdirection but also in an oblique direction when the polarizing plate ofthe present invention is used in a liquid crystal display apparatus. Thesecond protective layer may be isotropic depending upon the purpose.

Preferably, the slow axis direction of the second protective layer issubstantially parallel or perpendicular to the absorption axis directionof the polarizer. The polarizing plate having such a positionalrelationship of optical axes is used preferably in a normally black modeliquid crystal display apparatus.

In one embodiment, when the polarizing plate of the present invention isused in a liquid crystal cell having a liquid crystal layer alignedhomeotropically as in a vertical alignment mode, a refractive indexellipsoid of the second protective layer preferably satisfies arelationship: nx≧ny>nz. In this case, the slow axis direction of thesecond protective layer preferably is substantially perpendicular to theabsorption axis direction of an adjacent polarizer. Further, in thiscase, the Nz coefficient of the second protective layer is 0.9 to 4. Inanother embodiment, when the polarizing plate of the present inventionis used in a liquid crystal cell having a liquid crystal layer alignedhomogeneously as in an in-plane switching mode, a refractive indexellipsoid of the second protective layer preferably satisfies arelationship: nx≧nz>ny. In this case, the slow axis direction of thesecond protective layer preferably is substantially perpendicular to theabsorption axis direction of an adjacent polarizer. Further, in thiscase, the Nz coefficient of the second protective layer is −0.1 to 0.9.

As a material for forming the second protective layer, any suitable onemay be selected. Preferably, the second protective layer contains atleast one resin selected from the group consisting of a cellulose-basedresin, a norbornene-based resin, a polyimide-based resin, apolyester-based resin, and an acrylic resin. In the presentspecification, the “resin” may be a single polymer obtained from onekind of monomer or a copolymer obtained from at least two kinds ofmonomers. The second protective layer contains preferably 60 parts byweight to 100 parts by weight of the resin based on 100 parts by weightof the total solid content.

The cellulose-based resin may be obtained, for example, by a methoddescribed in JP07-112446A. The norbornene-based resin may be obtained,for example, by a method described in JP 2001-350017 A. Thepolyimide-based resin may be obtained by a method described in U.S. Pat.No. 5,344,916. The polyester-based resin may be obtained, for example,by a method described in U.S. Pat. No. 6,964,795. The acrylic resin maybe obtained, for example, by a method described in JP 2004-198952 A.

As a method of forming the second protective layer, any appropriateforming method may be employed. Examples of the forming method includecompression forming, transfer forming, injection forming, extrusionforming, blow forming, powder forming, FRP forming, and solvent casting.

As the second protective layer, a commercially available film may beused as it is. Alternatively, a commercially available film subjected tosecondary treatment such as stretching treatment and/or shrinkingtreatment may be used. Examples of the commercially available filminclude FUJITAC series (ZRF80S, TD80UF, TDY-80UL (trade name))manufactured by Fuji Photo Film Co., Ltd., “KC8UX2M” (trade name)manufactured by Konica Minolta Opto, Inc., ZEONOR series manufactured byOptes Inc., and ARTON series manufactured by JSR Corporation.

<E. Production Method>

As a method of producing a polarizing plate of the present invention,any suitable method may be selected. In one embodiment, it is preferredthat the polarizing plate of the present invention be produced by theproduction method including the following steps A₁ to D₁.

Step A₁: the step of preparing a solution that contains a lyotropicliquid crystal and a solvent and exhibits liquid crystallinity.

Step B₁: the step of applying the solution prepared in the step A₁ onthe surface of a long base in one direction to form a solidified layeror cured layer of the lyotropic liquid crystal.

Step C₁: the step of forming a mixed resin containing the thermoplasticresin layer (A) and the thermoplastic resin layer (B) into a long film,and stretching the film so that a transmission axis is expressed in alongitudinal direction.

Step D₁: the step of laminating the long base obtained in the step B₁and the long film obtained in the step C₁ so that the solidified layeror cured layer of the lyotropic liquid crystal is sandwiched by the baseand the film, and the longitudinal directions are matched with eachother.

According to such a production method, the polarizer having atransmission axis in a longitudinal direction and the first protectivelayer having a transmission axis in a longitudinal direction may beattached to each other by roll-to-roll, so that the productivity of apolarizing plate may be enhanced greatly.

In another embodiment, the polarizing plate of the present invention isproduced preferably by a production method including the following stepsA₂ to C₂.

Step A₂: the step of stretching a long film of a polyvinyl alcohol-basedresin containing a dichroic pigment in a longitudinal direction.

Step B₂: the step of forming a mixed resin containing the thermoplasticresin layer (A) and the thermoplastic resin layer (B) into a long film,and stretching the film so that a transmission axis is expressed in awidth direction.

Step C₂: the step of laminating the long film obtained in the step A₂and the long film obtained in the step B₂ so that the longitudinaldirections are matched with each other.

According to such a production method, the polarizer having atransmission axis in a width direction and the first protective layerhaving a transmission axis in a width direction may be attached to eachother by roll-to-roll, so that the productivity of a polarizing platemay be enhanced greatly.

<F. Other Layers>

The polarizing plate of the present invention may further include anyconstituent member. Referring to FIG. 1, any member may be provided on aside of the second protective layer 22 opposite to a side having thepolarizer 10, or on a side of the second protective layer 21 opposite toa side having the polarizer 10.

In one embodiment, an adhesion layer is further provided on a side ofthe second protective layer 22 opposite to a side having the polarizer10. When the polarizing plate with the adhesion layer is used, forexample, in a liquid crystal display apparatus, the adhesion layer isused for attachment of the polarizing plate to the surface of a liquidcrystal cell so that the second protective layer is opposed to theliquid crystal cell.

Although not limited, the adhesion layer preferably has apressure-sensitive adhesive layer. Generally, a liquid crystal displayapparatus is inspected before its shipment. At this time, if defects arefound in a polarizing plate itself or foreign matter is put between apolarizing plate and a liquid crystal cell, the polarizing plate ispeeled off so as to re-use the liquid crystal cell (which is also called“rework”). The pressure-sensitive adhesive layer is preferably excellentin adhesion and peelability so as to enable rework.

Preferably, the pressure-sensitive adhesive layer contains apressure-sensitive adhesive that may be obtained by cross-linking acomposition containing at least a (meth)acrylate-based (co) polymer, across-linking agent containing a compound with an isocyanate group as amain component, and a silane coupling agent.

In another embodiment, a retardation layer is further provided on a sideof the second protective layer 22 opposite to a side having thepolarizer 10. For example, in the case of using the polarizing plate ofthe present invention in a liquid crystal display apparatus, arefractive index ellipsoid of the retardation layer is appropriately setdepending on a refractive index ellipsoid of a liquid crystal cell to bemounted on the liquid crystal display apparatus. For example, when thepolarizing plate of the present invention is used for a liquid crystalcell (in which a refractive index ellipsoid exhibits a relationship:nz>nx=ny) as in a vertical alignment mode, a refractive index ellipsoidof the retardation layer preferably satisfies a relationship: nx=ny>nz(that is, the retardation layer is a negative C plate). Alternatively,when the polarizing plate of the present invention is used for a liquidcrystal cell (in which the alignment direction of liquid crystalmolecules changes gradually in a thickness direction) as in a bendnematic mode or a twisted nematic mode, the retardation layer ispreferably a positive O plate or a negative O plate.

<G. Liquid Crystal Display Apparatus>

The polarizing plate of the present invention is preferably used in aliquid crystal display apparatus. The liquid crystal display apparatusis used, for example, for: OA devices such as a personal computermonitor, a laptop personal computer, and a copying machine; portabledevices such as a cellular phone, a watch, a digital camera, a personaldigital assistance (PDA), and a portable game machine; householdelectric appliances such as a video camera, a television, and amicrowave oven; in-car devices such as a back monitor, a car navigationsystem monitor, and a car audio; display devices such as an informationmonitor for commercial stores; security devices such as a surveillancemonitor; and nursing care/medical devices such as a nursing monitor anda medical monitor.

The liquid crystal display apparatus of the present invention ispreferably used for a television. The television has a screen size ofpreferably wide 17-type (373 mm×224 mm) or more, more preferably wide23-type (499 mm×300 mm) or more, and particularly preferably wide32-type (687 mm×412 mm) or more.

EXAMPLES

The present invention will be described in more detail by using thefollowing examples and comparative examples. However, the presentinvention is not limited to the examples. Analysis methods used in theexamples are described below.

(1) Transmittance and Polarization Degree of a Polarizer:

The transmittance (T) and polarization degree were determined by using aspectrophotometer “DOT-3” (product name, manufactured by Murakami ColorResearch Laboratory). The transmittance (T) is a Y value obtained byundergoing luminosity factor correction with Two Degree Field (C lightsource) under JIS Z 8701-1982.

(2) Method of Measuring a Dichroic Ratio (DR):

Setting the complete polarization obtained through a Glan-Thompsom prismpolarizer to be 100%, transmittances: k₁ and k₂ with respect to eachlinearly polarized light were obtained using an integrating spherespectrophotometer (“U-4100” (product name), manufactured by HitachiLtd.). A single axis transmittance (Ts) was calculated by an expression:Ts=(k₁+k₂)/2. A dichroic ratio (DR) was calculated by an expression:DR=log(1/k₂)/log(1/k₁). Herein, k₁ represents a transmittance oflinearly polarized light in a maximum transmittance direction, and k₂represents a transmittance of linearly polarized light in a directionperpendicular to the maximum transmittance direction.

(6) Method of Measuring a Thickness:

A thickness of less than 10 μm was measured by using a thin filmthickness spectrophotometer “Multichannel photodetector MCPD-2000”(trade name, manufactured by Otsuka Electronics Co., Ltd.). A thicknessof 10 μm or more was measured by using a digital micrometer“KC-351C-type” (manufactured by Anritsu Corporation).

(3) Method of Measuring Retardation Values (Re[λ], Rth[λ]), NzCoefficient, and T[590]:

Measurement was performed at 23° C. by using “KOBRA21-ADH” (trade name,manufactured by Oji Scientific Instruments). As the average refractiveindex, used was a value determined by using an Abbe refractometer“DR-M4” (product name, manufactured by Atago Co., Ltd.).

(8) Method of Measuring an Absolute Value (C[590]) of a PhotoelasticCoefficient:

The retardation values (23° C./wavelength of 590 nm) at the center of asample having a size of 2 cm×10 cm were measured under stress (5 to 15N) by using a spectroscopic ellipsometer “M-220” (product name,manufactured by JASCO Corporation) while both ends of the sample wereheld, and the absolute value (C[590]) of a photoelastic coefficient wascalculated from a slope of a function of the stress and the retardationvalues.

Production of Polarizer Reference Example 1

A polyvinyl alcohol film (“VF-PS#7500” (product name), manufactured byKuraray Co., Ltd.) with a thickness of 75 μm was swollen with pure waterat 30° C., and thereafter, stretched so that the final stretchingmagnification became 6.2 times the original length of the film whilebeing dyed in an aqueous solution containing 0.032 parts by weight ofiodine and 0.2 parts by weight of potassium iodide with respect to 100parts by weight of water. The stretched film was dried for one minute at40° C. in an air circulation type dry oven. A polarizer A thus obtainedhad a thickness of 25 μm, a transmittance of 42.6%, and a polarizationdegree of 99.99%.

Reference Example 2

An aqueous solution containing a dichroic pigment (“LC polarizer”(product name), manufactured by Optiva Inc.) containing a sulfonatedperylene-based pigment and pure water, in which the concentration of thedichroic pigment was adjusted to 12.2% by weight, was applied to thesurface of a triacetylcellulose film with a bar coater, followed bynatural drying, whereby a polarizer was produced on the surface. Apolarizer B thus obtained had a thickness of 0.2 μm, a single axistransmittance of 42.1%, and a dichroic ratio (at 600 nm) of 35. Theaqueous solution exhibited a nematic liquid crystal phase at 23° C. whenobserved with a polarization microscope.

Production of First Protective Layer Reference Example 3

A mixed resin containing 75% by weight of polyethylene naphthalate and25% by weight of syndyotactic polystyrene was subjected to fusionextrusion to form a film having a thickness of 600 μm. The obtained filmwas stretched in a width direction at 135° C., using a polyester filmtenter. The film (first protective layer A) thus obtained had areflection axis in a width direction and a transmission axis in amechanical direction, a thickness of 400 μm, R_(1x)[590] of 73%, andreflectance (R_(1x)[590]) in a transmission axis direction of 35%.

Production of Second Protective Layer Reference Example 4

A polymer film [“Zeonor ZF14-100” (product name), manufactured by OptesInc.] containing a norbornene-based resin having a thickness of 100 μmwas stretched 2.7 times in an air circulation typetemperature-controlled oven at 150° C. by a fixed-end transverseuniaxial stretching method (a method of stretching a film in a widthdirection while fixing the film in a longitudinal direction), using atenter stretching machine. The retardation film (second protective layerA) thus obtained had a refractive index ellipsoid exhibiting arelationship: nx>ny>nz, a thickness of 35 μm, T [590] of 91%, Re[590] of120 nm, Rth[590] of 160 nm, Nz coefficient of 1.33, and C[590] of5.1×10⁻¹² m²/N.

Production of a Retardation Layer Reference Example 5

The polyimide powder (6FDA/TFMB) was dissolved in methyl isobutyl ketoneto prepare 15% by weight of a polyimide solution. The polyimide solutionwas cast uniformly in a sheet shape on the surface of a triacetylcellulose film [“TD80UF” (product name), manufactured by Fuji Photo FilmCo., Ltd] having a thickness of 80 μm with a slot die coater. Next, thefilm was provided in an air circulation type dry oven having amulti-chamber, dried at 80° C. for 2 minutes, at 135° C. for 5 minutes,and at 150° C. for 10 minutes, and finally the triacetyl cellulose filmwas peeled. The polyimide layer (retardation layer A) thus obtained hada refractive index ellipsoid exhibiting a relationship: nx=ny>nz, athickness of 5 μm, T[590] of 90%, Re[590] of 1 nm, and Rth[590] of 210nm.

Production of a Polarizing Plate Example 1

The first protective layer A obtained in Reference Example 3 wasattached to one side of the polarizer A produced in Reference Example 1via a water-soluble adhesive containing a polyvinyl alcohol-based resin(“GOHSEFIMER Z200” (product name), manufactured by Nippon SyntheticChemical Industry Co., Ltd.) so that the transmission axis directionthereof became substantially parallel to the transmission axis directionof the polarizer A. Next, the second protective layer A produced inReference Example 4 was attached to the other side of the polarizer Avia the water-soluble adhesive so that the slow axis direction thereofbecame substantially perpendicular to the absorption axis direction ofthe polarizer A. Next, the retardation layer A obtained in ReferenceExample 5 was attached to a side of the second protective layer Aopposite to the side having the polarizer A via an acrylicpressure-sensitive adhesive. The polarizing plate thus obtainedexhibited a contrast ratio higher than that of a conventional polarizingplate, when used in a liquid crystal display apparatus.

INDUSTRIAL APPLICABILITY

As described above, the polarizing plate of the present invention canexhibit a high contrast ratio in the case of being used in a liquidcrystal display apparatus, so the polarizing plate is significantlyuseful for enhancing the display properties of, for example, a liquidcrystal television, a personal computer monitor, and a cellular phone.

1. A polarizing plate, comprising: a polarizer; a first protective layerprovided on one side of the polarizer; and a second protective layerprovided on the other side of the polarizer, wherein the firstprotective layer has a function of separating incident light into twopolarized light components perpendicular to each other, transmitting onepolarized light component, and reflecting the other polarized lightcomponent.
 2. A polarizing plate according to claim 1, wherein atransmission axis direction of the polarizer and a transmission axisdirection of the first protective layer are substantially parallel toeach other.
 3. A polarizing plate according to claim 1, wherein thepolarizer comprises a stretched film of a polyvinyl alcohol-based resincontaining a dichroic pigment.
 4. A polarizing plate according to claim1, wherein the polarizer comprises a solidified layer or a cured layerof an aligned lyotropic liquid crystal.
 5. A polarizing plate accordingto claim 4, wherein the lyotropic liquid crystal contains an azo-basedpigment, an anthraquinone-based pigment, a perylene-based pigment, anindanthrone-based pigment, an imidazole-based pigment, or a mixturethereof.
 6. A polarizing plate according to claim 1, wherein the firstprotective layer comprises a laminate containing a thermoplastic resinlayer (A) and a thermoplastic resin layer (B).
 7. A polarizing plateaccording to claim 1, wherein the first protective layer comprises thethermoplastic resin layer (A) and the thermoplastic resin layer (B)provided alternately.
 8. A polarizing plate according to claim 6,wherein the thermoplastic resin layer (A) substantially exhibitsanisotropy, and the thermoplastic resin layer (B) substantially exhibitsisotropy.
 9. A polarizing plate according to claim 6, wherein anin-plane birefringent index (Δn_(xy)[590]) at a wavelength of 590 nm ofthe thermoplastic resin layer (A) is 0.05 or more.
 10. A polarizingplate according to claim 6, wherein a refractive index (ny_(A)) in afast axis direction of the thermoplastic resin layer (A) and arefractive index (ny_(B)) in a fast axis direction of the thermoplasticresin layer (B) are substantially the same.
 11. A polarizing plateaccording to claim 6, wherein the thermoplastic resin layer (A) containsa polyethylene terephthalate-based resin, a polytrimethyleneterephthalate-based resin, a polybutylene terephthalate-based resin, apolyethylene-naphthalate-based resin, a polybutylene naphthalate-basedresin, or a mixture thereof.
 12. A polarizing plate according to claim6, wherein the thermoplastic resin layer (B) contains apolystyrene-based resin, a polymethyl methacrylate-based resin, apolystyrene glycidyl methacrylate-based resin, or a mixture thereof. 13.A polarizing plate according to claim 1, wherein an in-plane retardationvalue (Re[590]) at a wavelength of 590 nm of the second protective layeris 10 nm to 400 nm.
 14. A polarizing plate according to claim 1, whereina thickness direction retardation value (Rth[590]) at a wavelength of590 nm of the second protective layer is 10 nm to 800 nm.
 15. Apolarizing plate according to claim 1, wherein the second protectivelayer contains at least one kind of resin selected from the groupconsisting of a cellulose-based resin, a norbornene-based resin, apolyimide-based resin, a polyester-based resin, and an acrylic resin.16. A polarizing plate according to claim 1, further comprising anadhesion layer on a side of the second protective layer opposite to aside having the polarizer.
 17. A polarizing plate according to claim 16,further comprising a retardation layer on a side of the secondprotective layer opposite to a side having the polarizer.
 18. Apolarizing plate according to claim 17, wherein the retardation layerhas a function of compensating a liquid crystal cell optically.
 19. Aliquid crystal display apparatus comprising the polarizing plateaccording to claim 1.